The present disclosure is directed at spinal implants and methods for the preservation of implant function.
Whether for degenerative disease, traumatic disruption, infection or neoplastic invasion, surgical reconstructions of the bony skeleton are common procedures in current medical practice. Regardless of the specifics of the individual operation, many surgeons employ bone grafts and an implantable device to bridge the diseased segment and provide structural support for the remaining skeleton. While the device provides immediate support, long term stability is critically dependant on the formation of a bony bridge across the defect. Using this strategy, diseased segments within the spinal column are similarly repaired using bone grafts and implantable devises. These implants are especially useful in spinal surgery where they can restore spinal alignment and provide immediate stability for the spinal column.
The end result of these operative procedures is bony fusion. That is, a segment of continuous bone is formed between the spinal segment above and below the diseased region. Bony fusion reconstitutes the load bearing capability of the spinal column but destroys the segmental mobility that is characteristic of normal spinal function. Consequently, segmental fusion alters the balance of forces across the spine and necessarily increases the forces acting upon the motion segments above and below the fused region. These load alterations are significant and will accelerate the formation of degenerative changes within the adjacent segments. With time, these segments will also require fusion.
The increase in the rate of degeneration at the spinal segments adjacent to a fused segment has been termed “adjacent segment disease” and presents a significant clinical problem. Approximately 30% of patients who undergo spinal fusion will require fusion of an adjacent segment within 10 years of the original operation. In turn, the extended fusion will redistribute force across neighboring segments and lead to their degeneration, thereby setting up a vicious cycle whereby bony fusion begets additional fusion.
To address this growing problem, there has been interest in devices that can span the diseased spinal region and recreate the motion characteristics of the normal spine. These efforts at motion preservation have lead to the recent introduction of artificial disc devices capable of mimicking the normal movements of the intra-vertebral disc. Surgical implantations of these artificial discs have yielded promising results in both US and European trials. However, the growing experience with these implants has also uncovered factors that complicate the surgical procedure and can lead to premature device failure.
Since the mobile implants are larger and must be placed more precisely than fusion devices, the surgical implantation procedure is necessarily more demanding. An optimal access route to the spine must be used because an indirect approach will only add to the technical difficulty. Since the spinal cord and/or spinal nerves lie behind the vertebral bodies, an anterior approach to the spine provides the most direct and unfettered access to the vertebral disc space. Understandably, all “artificial” discs in current use require placement through an anterior approach.
There is extensive experience with anterior spinal surgery from the current placement of fusion devices and a general recognition of the potential risks inherent in this approach. Since the spinal column is situated posteriorly within the body, an anterior approach requires dissection through the many other structures that lie anterior to the spine. This has proven most challenging in the chest and abdomen where the body's largest blood vessels, the aorta and vena cava, lie immediately anterior and lateral to the spine. Nevertheless, growing surgical experience has reduced the risk to these vessels and other thoraco-abdominal structures to acceptable levels with initial operation. However, scar formation greatly increase the risk of re-operation. With estimates of major vascular injury rates at 30%, the risk of mortality or significant morbidity at second operation is high. For this reason, surgeons currently address a failed anterior fusion by applying a posterior approach at re-operation and thereby avoid the prohibitive risks of recurrent anterior surgery.
The difficulties with re-operative anterior spinal fusion surgery are magnified when motion preservation devices are used. Since these devices are larger than fusion implants, a larger dissection field is needed to place them and the increased dissection leads to a wider region of scar formation. In fusion surgery, the implanted device immobilizes the spine and bears load until the bone graft has healed. Once fused, the newly formed bone effectively shields the implant and, consequently, time-dependant implant fatigue does not occur. However, devices that recreate spinal mobility are designed to replicate complex movement in various planes and are generally implanted in younger patients than the fusion group. They must withstand millions of cycles of repetitive loading as well as endure significant moment arms and shear forces. While fusion devices are expected to withstand those forces until bone fusion occurs, motion preservation devices will be subjected to these forces for the duration of their functional life. Consequently, some implants will dislodge, wear and fail. Since implant replacement through a posterior approach is not possible, patients with failed implants will be subjected to the significant risks of re-operative anterior surgery.
Motion preservation devices contain moving parts and scar in-growth into the device will interfere with proper movement and greatly increase the likelihood of implant failure. In addition, calcification within the scar tissue or within the disc space adjacent to the implant will create a fusion mass around the device and render it useless. Consequently, control of local scar formation, calcification and tissue growth into the implant is imperative. Failure to do so will greatly increase the likelihood of implant failure and require that patients be subjected to additional surgery with substantial risks.
Lastly, all moving components will inevitably produce wear debris and spinal motion preservation devices will also shed particulates. Experience from knee and hip prosthesis has shown that wear particles can lead to bone breakdown and implant loosening, can produce local tissue inflammation and toxicity, and can disseminate through the blood stream to distant organs. Consequently, limitation and containment of the wear debris is important in biological implants. It is even more important in devices placed adjacent to the nervous system, such as spinal implants.
U.S. Pat. Nos. 6,673,362; 6,531,146; 6,521,223; 6,294,202; 6,235,726; 6,010,692 and 5,795,584 all disclose methods for the attenuation of scar formation during post-operative healing. These and other prior art patents describe various compounds, agents and methods that decrease adhesions between two or more tissues. None of these patents teach the use of the agents and methods to prevent adhesions between a tissue and a movable implant, to minimize tissue invasion into the implant, to inhibit bone formation within tissue adjacent to the implant, or to contain wear debris shed by the implant.